Diode-pumped double frequency solid-state laser

ABSTRACT

In a diode-pumped frequency-doubled solid-state laser, all the elements of the resonator as well as the elements inside the resonator such as the laser crystal, mirror, and frequency doubler, are accommodated in or on an angle or U profile, which is milled as a solid part from a solid piece or is manufactured as a cast, stamped, or sintered part from metal or ceramic. The profile has one or more intermediate walls that are made of the same piece, as the angle or U profile to lend additional stability. The elements of the resonator or elements inside the resonator are contained on or in the resonator, and the intermediate walls have openings to allow the laser mode and/or the laser irradiation to pass through.

The invention relates to a diode-pumped, frequency-doubled solid-statelaser according to the preamble of Claim 1.

Solid-state lasers, usually built using rare-earth-doped crystals orglasses, for example Nd:YAG, Nd:YVO₄, Nd:YAlO, ND:YLF, Nd:glass or othersimilar solid materials and equipped with resonator-internal frequencydoubling, have been known for a long time and are used in manyapplications in laser technology. Generation of second or higherharmonic oscillations is used in materials, primarily crystals, whichhave no inversion centers—for example KTP, LBO, BBO, KNbO₃, LiNbO₃ orothers—with a high nonlinear coefficient, which generates light at twiceor four times the frequency of the radiant energy received, byinharmonic oscillations of the lattice atoms, excited by an incidentlight wave. The process of generating higher harmonics is highlydependent upon the power density (see for example Köchner, Solid-StateLaser Engineering) so that to produce frequency-doubled laser radiationwith higher efficiency, the non-linear crystal is usually, at least incontinuously operating (cw) lasers, accommodated in the resonator of thelaser itself (see above or also for example Yariv, Quantum Electronics,third edition, Page 402). The resonator mirrors are usually chosen to behighly reflecting for the laser wavelength in order to achieve a maximummagnification in the resonator and hence a doubling efficiency that isas high as possible. The decoupling mirror is simultaneously highlytransmitting for the frequency-doubled radiation in order to be able todecouple the latter readily from the resonator.

Usually such lasers are built on optical benches, in other words theelements for holding the laser crystal, frequency doubler, anddecoupling mirror are usually screwed tight on the underside to a plateor lined up along a rail by means of displaceable structures. Thisdesign however, in cases of resonator-internal frequency doubling inwhich an especially slight adjustment tolerance is necessary because ofthe power-density-dependent conversion efficiency, is not sufficientlystable in the long term when exposed to changing environmentalconditions or over a long operating life. In particular, bending of theoptical bench or rail as well as tilting of the retaining elements thatare fastened on only one side are responsible for this.

One known alternative design provides for mounting the retainingelements on three or for steel rods which are typically inserted throughmatching openings in the corners of the (usually rectangular) retainingelements, with the retaining and adjusting elements being secured byclamps to the rods. Although no significant tilting of the retainingelements with respect to one another can occur, practice shows that whenthe retaining elements are clamped to the rods, stresses are exerted onthe rods which likewise lead to the elements going out of adjustment inthe long term. In addition, good heat transfer from the holders is notpossible, with the holders being subjected to heat for example from thelaser crystal or the frequency doubler, since the rods have a smallcross section and therefore a poor conductivity, as well as no possiblecontact with a larger thermal mass serving as the cooling body.

Yet another solution according to the prior art is provided by mountingthe resonator elements either in threaded sleeves, with no independentretaining structure being formed and with the stability of an elementdepending on the stability of the mounting elements, or in tubes, whichallows the resonator to be manufactured only in a certain sequence andnot permitting subsequent removal of a central resonator element.

Finally, retaining and adjusting elements can be clamped displaceably ona (thicker) tube instead of a plate, which has the same disadvantages asthe optical bench as far as tilting and heat removal are concerned.

Therefore the goal of the invention is to provide a resonator structurewhich is free of stress, has no internal twisting or tensioning by theretaining or adjusting elements, exhibits good heat transfer and theproperties of a cooling body, as well as a large contact surface toreceive retaining elements subject to thermal stress.

This is achieved by the features listed in the characterizing clause ofClaim 1. Details of the invention will be found in the subclaims and thespecification, in which several embodiments are explained with referenceto the drawing.

FIG. 1 is a sketch of the arrangement according to the invention of aresonator structure with the resonator elements mounted;

FIG. 2 is a perspective view of the resonator structure according to theinvention without the resonator elements;

FIG. 3 shows an external laser housing with a dust-proofed resonatorstructure attached as a subchassis;

FIG. 4 is a sketch of the decoupling unit for power and noisemeasurement, and

FIG. 5 is an arrangement according to the invention of a two-dimensionalresonator arrangement using the example of a V-resonator.

The solution according to the invention provides a design which consistsof an angle or U profile which is either milled from a single piece oflow-stress material, for example low-stress aluminum, by milling forexample or is produced as a cast part free of stress in one piece. Thisangle or U profile also contains at least one, but preferably at leasttwo, intermediate walls that are made from the same piece and thereforeprovide the angle with additional stability against twisting andtension. Such an arrangement, always relative to the case of an angle,is shown in the following FIGS. 1 to 5.

FIG. 2 for example shows the angle (1) with intermediate walls (7 a) and(7 b) that are machined from one piece. Intermediate walls (7 a) and (7b) have openings that permit unimpeded propagation of the resonator modeor the laser radiation along the axis of the angle. On the front andback, closing off the angle, either walls likewise made of the samepiece or, as in the drawing, bolted plates (4) and (12) can be providedthat lend the angle additional stability.

Flush retaining and adjusting elements can then be mounted on theseretaining plates and intermediate walls, for example by bolting. As aresult, maximum stability of these elements with respect to one anotheris guaranteed as well as a good removal of heat from any heat sources asa result of flush contact with the intermediate wall. In this fashion,the angle simultaneously serves as a thermal mass and a cooling body,with the coolable thermal load being limited in particular by theresultant length expansion and possible lack of adjustment of theelements, which however is minimized in this arrangement.

This mounting of a resonator element is shown clearly in FIG. 1. A lasercrystal (6) is mounted in the first intermediate wall (7), said crystalbeing held in a bushing (5). The material of this bushing is chosen sothat the temperature of the laser crystal is optimum and so that as aresult of the induced thermal lens, the resonator mode is focused at thelocation of the frequency-doubling crystal (8) and therefore results inan increased power density, from which a higher frequency conversionfollows. Frequency doubler (8) in turn is received in a holding andadjusting device 9 that permits adjustment perpendicularly to theradiation direction (this is an axis that passes through all opticalelements (6), (8), and (11) in two directions as well as tilting in twodirections perpendicularly to the beam axis as well as rotation aroundthe beam axis. In this manner, both the phase adjustment angle and thepolarization direction and the optical perpendicular position of thepistol can take place. The frequency doubler can also be heated orcooled by a Peltier element 10, with the heat from the Peltier elementbeing transmitted for example to the retaining and adjusting device 9and the intermediate wall (7 b); alternative possibilities for heattransport are also possible of course.

At the right-hand end of the angle in FIG. 2 is a bolted plate 12 whichcan also be made as the end wall of an angular body manufactured fromone piece like walls (7 a) and (7 b). A retaining and adjusting device16 is mounted on this wall, said device supporting laser mirror 11 andpermitting the latter to tilt as well as to shift perpendicularly withrespect to the beam axis in two directions. At the left end of theangle, a plate (4) is likewise mounted, said plate likewise possiblybeing a fixed wall of the angle on which a lens (3) for focusing thepumped light radiation is mounted by a retaining and adjusting device14. This lens (3) is in a sleeve 2 that on one side has a helical threadto receive a light guide through which the pumped light radiation iscoupled to the laser crystal. The side of the laser crystal (6) thatfaces away from laser mirror (11) has an optical coating (17) thatserves as a second laser mirror and simultaneously transmits the pumpedlight. Both mirrors (17) and (11) are preferably designed to be highlyreflecting for the basic wavelength, with coating (17) being highlytransmitting for the frequency-doubled laser radiation as well. Coating(17) is also highly transmitting for the pumped light radiation and istransmitting or reflecting for the frequency-doubled laser radiation.

Openings (13 a, 13 b), . . . are provided in angle (1) which allowelectric contacts to be fed through, for example for the Peltier element(10) or for temperature sensors that are connected with the frequencydoubler or the laser crystal.

As indicated in FIG. 2, the angle can be sealed by a dust-proofing hood(18) so that no dust can penetrate the interior of the resonator. Thedust-proofing hood (18) can be either provided with the sealing lips onthe contact surfaces with angle (1) or it can be bent accordingly toguarantee optimal protection against dust. In order to permit theresonator elements to be adjusted even when the dust-proofing hood isclosed, openings (18 a), (18 b), (18 c) . . . can be made in the hoodthat match the respective adjusting screws of retaining devices (4),(9), or (16) and allow an adjusting tool to be inserted. When adjustmentis complete, these openings are sealed by labels, plugs, or like.

The angle (1) that carries the resonator is machined on the bottom sothat it rests exactly on three small surfaces or points on a flatsurface so when it is fastened to a plate or the like, the angle is notsubjected to tension.

The resonator structure (26) with this design and protected againstdust, as shown in FIG. 3, is accommodated in a housing that fits over itin which, in addition to resonator (26), additional elements requiredfor the laser such as pc boards (24) and beam diagnosis devices (23)(for example for measuring the laser power or the laser noise) arelocated. The housing that fits on top consists of a bottom plate (20), afront plate (27), a back plate (19), and a lid (25). All parts of thehousing are preferably made of conducting material (aluminum forexample) so that good shielding against electromagnetic radiation isprovided for the laser and the electronics. Electrical connections (22)for the pc boards can be provided in the front or rear walls. Inaddition, a light guide (28) is fed through one of the walls of thehousing, said guide being connected to bushing (2). In order to protectthe light guide from being disassembled from outside and to guarantee asolid connection to the laser that cannot be broken from the outside, abushing (21) is guided over the end of the light guide and screwedtightly to the housing (back plate (19) for example) so that the screwconnection of the light guide itself is not accessible from outside.

In many cases, it is necessary to guide a portion of the laser radiationthrough a photodiode of a pc board for power stabilization, noiseregulation, etc. For this purpose, in the system described, the beamdiagnosis device (23) is designed as described in FIG. 4.

The laser beam emerging from resonator (26) strikes a beam splitterplate (32) which is coated so that a portion (31) of the beam isreflected but the majority (30) passes through the beam splitter plate.The coating is designed so that depending on the position of thepolarization of the laser radiation, the degree of reflection can bevaried by rotating the plate. The decoupled beam (31) initially strikesa filter (33) that separates the laser radiation from other radiation bywavelength selection (dielectric filter or absorption filter). Then thebeam strikes a plate or film (24) that serves as a scatter disk. Thedisk is preferably made of ceramic or Teflon which produces especiallygood scattering of the beam. The scattering is necessary in order toblur inhomogeneities in the beam (31) and simultaneously to preventfluctuations in intensity caused by a coherent effect (speckles) so thatno artificial fluctuations occur at the location of the photodiode (38).The photodiode for example is mounted on a board (35) that is connectedwith the beam diagnosis device by a retaining screw (36). The electricalcontact to the pc board (24) is made through the board and a cable.

Beam diagnosis unit (23), for precise adjustment of a defined level atthe photodiode (38) on the front panel (27), is located both rotatably(because of the polarization-dependent reflection of the beam splitterplate (33)) and also displaceably in two axes perpendicular to the beamaxis.

In addition to the elements described, additional elements can beaccommodated in the housing that consists of (25), (20), (27), and (19)either in or outside the beam path. It is advantageous for example (forreasons of clarity not shown as in FIG. 3) to mount a lens or a lensarrangement in laser beam (29) or (30) to expand or focus a laser beam,which can be displaced perpendicularly to the optical axis in twodirections in order to permit correction of the beam alignment relativeto the housing and/or a plane plate that permits correction of the beamheight and the lateral offset during rotation around two axesperpendicular to the optical axis. Both functions, lateral offsetcorrection and beam alignment, can be also performed by a wedge platelocated in the beam (29) or (30), which is displaceable as well asrotatable in two directions perpendicularly to the optical axis.

Finally, it should be noted that, as mentioned at the outset, inaddition to the angles shown in the drawings, U profiles can also beused. It should also be pointed out that the angles or U profiles can beused not only in linear arrangements in which the structures that have anon-linear, for example a two-dimensional arrangement (for example a Vor Z arrangement), but also for example in an arrangement for a Vresonator as sketched in FIG. 5.

What is claimed is:
 1. A diode-pumped frequency-doubled solid-statelaser, wherein all elements of a resonator as well as elements insidethe resonator are mounted in or on an angle or U profile milled as asolid part from an entire piece or manufactured as a cast, stamped, orsintered part from metal or ceramic, and in that said profile has one ormore intermediate walls made from the same piece and mounted like theangle or U profile to lend additional stability to the profile, furtherwherein the resonator or resonator-internal elements are mounted in thesame element and that the intermediate walls have openings to allow atleast one of the laser mode and laser radiation to pass though. 2.Diode-pumped frequency-doubled solid-state laser according to claim 1wherein a solid-state laser crystal is secured in a bushing made ofaluminum, copper, steel, or ceramic, which is received in an opening ofone of the intermediate walls of the angle or U profile, with thematerial of the bushing being chosen for its heat conducting property sothat the laser crystal, when optically excited by pumped light, assumesa temperature that is favorable for operation.
 3. Diode-pumpedsolid-state laser according to claim 2, wherein the temperature of thecrystal is chosen so that the thermal lens produced by the radialtemperature gradients produces a focusing of the resonator mode at thelocation of the frequency doubler.
 4. Diode-pumped solid-state laseraccording to claim 1, wherein a laser crystal, on a side facing awayfrom a frequency doubler, has an optical coating that functions as alaser mirror.
 5. Diode-pumped solid-state laser according to claim 1,wherein a frequency doubler is received in a holder that permitsadjustment of the crystal in the two lateral directions perpendicular tothe beam path of the laser, as well as a tilting around each of theseaxes and a rotation around the beam axis, and is connected permanentlyby this holder with an intermediate wall of the angle or U profile ofthe resonator.
 6. Diode-pumped solid-state laser according to claim 1,wherein a frequency doubler can be temperature-regulated and/orstabilized by a Peltier element.
 7. Diode-pumped solid-state laseraccording to claim 1, wherein a decoupling mirror of the laser isreceived in a holder which can be displaced in two directionsperpendicular to the beam axis and can be tilted around this axis and issecured by this holder with an intermediate wall or by a retaining platewith one end face of the angle or U profile.
 8. Diode-pumped solid-statelaser according to claim 1, wherein the angle or U profile supportingthe resonator is machined on its underside so that it rests at onlythree points on a flat surface.
 9. Diode-pumped solid-state laseraccording to claim 1, wherein the angle or U profile is provided with ahood that seals off the angle or U profile with the respective holdersfor the optical elements in such fashion that no dust can penetrate theenclosed interior of the resonator, with the hood being provided withbends or sealing lips on the respective boundary surfaces for sealing.10. Diode-pumped solid-state laser according to claim 9 wherein the hoodis provided with holes that permit a suitable tool to be inserted tochange the positions of the optical elements by the adjusting devices,with the holes being sealed for normal operation by suitable covers. 11.Diode-pumped solid-state laser according to claim 1, wherein theresonator structure composed of the angle or U profile with all of theadjusting and retaining devices and optical elements as well as adust-proofing hood is bolted or connected as a subchassis in a largerhousing, said housing representing an enclosed upper housing andreceiving in addition to the resonator structure, additional elements ofthe laser system, including at least one pc board for regulating andcontrolling the laser as well as elements for diagnosing laserradiation.
 12. Diode-pumped solid-state laser according to claim 11,wherein the outer housing of the laser consists of a conducting materialand that good shielding against external electromagnetic radiation isprovided for the laser and the electronic elements in the laser. 13.Diode-pumped solid-state laser according to claim 1, wherein openingsare machined in the angle or U profile, through which openingselectrical element such as Peltier elements for temperature control ortemperature sensors can be connected in the laser housing with anelectronic unit contained there.
 14. Diode-pumped solid-state laseraccording to claim 1, wherein a portion of the laser radiation isconducted through at least one beam splitter plate to a photodiode whichis connected with an electronic unit contained in a housing and thuspermits detection of power, power fluctuations, or amplitude noise. 15.Diode-pumped solid-state laser according to claim 14 wherein thephotodiode has a bandwidth of least 1 MHz.
 16. Diode-pumped solid-statelaser according to claim
 14. wherein the beam splitter plate is coatedso that the reflection is polarization-dependent and so the amount ofdecoupled light can be adjusted depending on the angle to thepolarization of the laser radiation.
 17. Diode-pumped solid-state laseraccording to claim 1, wherein a decoupling unit is mounted displaceablyin two directions perpendicular to the optical axis, so that it can beadjusted in the laser beam and mounted rotatably so that the amount ofdecoupled light can be adjusted by rotating a beam splitter platecontained in it.
 18. Diode-pumped solid-state laser according to claim1, wherein filters are mounted in front of a photodiode for selectingthe laser radiation from the rest of the light radiation and, inaddition, further wherein a scatter disk made of a diffuse material thatattenuates the light radiation and as a result of the scatter, blurs theintensity distribution over the beam cross section is mounted in frontof the photodiode so that no fluctuations in the spatial beam profile orfluctuations in intensity that also occur due to coherent effectsfalsify the photodiode signals.
 19. Diode-pumped solid-state laseraccording to claim 1, wherein a device is mounted to receive opticalelements on a beam outlet side, on the angle or U profile of theresonator or on an outlet side of a laser subchassis or of a surroundinglaser housing.
 20. Diode-pumped solid-state laser according to claim 1,wherein a lens is provided in an emerging laser beam to expand or focusthe laser beam and/or a lens or lens arrangement adjustable in twodirections perpendicular to the beam axis to change the exit angle ofthe laser radiation relative to the housing walls.
 21. Diode-pumpedsolid-state laser according to claim 1, wherein a flat plate isaccommodated in an emerging laser beam for correcting beam height andlateral offset of the laser beam relative to housing walls, said platebeing capable of being tilted around the two axes that are perpendicularto the beam axis.
 22. Diode-pumped solid-state laser according to claim1, wherein a wedge plate is accommodated in an emerging laser beam forcorrection of both an exit angle and a beam position of the laserradiation, said plate being capable of being tilted around two axesperpendicular to the beam axis and also capable of being displacedaround two axes perpendicularly to the beam axis.
 23. Diode-pumpedsolid-state laser according to claim 1, wherein pumping light from alaser diode for optical excitation of a laser crystal guided to thelaser crystal through a light guide and the radiation emerging from thislight guide is focused by a lens on the laser crystal, with the lightguide being connected by a screw device with the resonator unit andwhich is secured by a top-mounted sleeve tightly screwed to a laserhousing so that the light guide cannot be removed from outside withoutremoving the sleeve.
 24. Diode-pumped solid-state laser according toclaim 1, wherein a lens for focusing pumped light radiation can bedisplaced by a suitable adjusting device in two directionsperpendicularly to the beam axis and can also be tilted around two axesperpendicularly to the beam axis, with an additional adjustment of thedisplacement in the direction of the beam axis also being possible. 25.Diode-pumped solid-state laser according to claim 24 wherein alight-guide fiber is tightly screwed to a bushing that holds the lensand thus is displaced and tilted together with the lens. 26.Diode-pumped solid-state laser according to claim 1, wherein a bushingtogether with a lens and a light guide are permanently connected withthe angle or U profile by an adjusting device with a retaining plate orother intermediate wall of the angle or U profile.